Polycarbonate (PC) and polystyrene (PS) have long been under extensive research effort as a substitute for inorganic glass in the manufacturing of optical lenses. Some of the disadvantages of these polymers have been noted. More specifically, polycarbonate shows inferior surface hardness and the optical lenses made therefrom often present double or even multiple refraction problems. On the other hand, polystyrene polymers lack impact and heat resistance, and exhibit poor cutting processability.
More recently, another polymer has also been developed as a base material in making optical lenses. This polymer, which is prepared from diethyl glycol bisallyl carbonate (hereinafter referred to as "CR-39", which is a trade name used by PPG, Pittsburg, Pa.), has shown to be light-weight, safe, and with excellent processability and dyeability. These advantageous properties have made CR-39 a superior product than inorganic optical materials. CR-39 also has excellent stability and mechanical properties due to its crosslinking structure. As a result, CR-39 has been widely used as a substitute for inorganic glass in the manufacturing of optical lenses.
However, CR-39 has been observed to exhibit several undesirable properties such as undesirably high shrinkage factor (during hardening) as well as relatively low refractive index (n.sub.D =1.50). The volume of CR-39 shrinks by a factor of about 14% during the hardening step. To ensure that the final product conforms to the exact predetermined curvature during molding and avoid optical distortions, a prolonged polymerization time is often required for CR-39 based lenses, usually between about 15-20 hours. Such a long polymerization time results in increased overall manufacturing time and the accompanying operational complexity as well as production cost.
The process of using CR-39 in the manufacturing of optical lenses has been disclosed U.S. Pat. No. 4,146,696, issued to Bond. The lenses made from CR-39 have shown to exhibit excellent chemical stability and abrasion resistance, as well as tolerance to high temperature. However, as described above, the CR-39 lenses require long polymerization time and show relatively low refractive index.
In Japanese Pat. Pub. P1-185501 by Takamizaria, it is disclosed using a thiocontaining urethane resin obtained from the reaction product between polythiol and polyisocyanate in making optical lenses. This polymer shows inadequate heat resistance, and is susceptible to forming gas bubbles with water during the molding process; it often sticks to the mold and makes their release from the mold difficult. Furthermore, polyisocyanate is susceptible to reacting with polythiol at room temperature, thus making this lens manufacturing process difficult to control and resulting in low yield.